JULY 13, 2010
Eating to Live or Living to Eat?
Stomach vs. Brain: Discovering Why Some People Can Resist Dessert While
Others Can't
Imagine the typical office birthday party.
It's after lunch, so everybody is full. Then, in comes a luscious
chocolate confection. The sight, the smell—even the sound of the word
"cake!"—stimulate the reward-and-pleasure circuits of the brain,
activating memory centers and salivary glands as well.
Melinda Beck asks the age-old question: Do you eat to live, or live to eat?
Scientists, it turns out, have learned much more about how appetite works
in the brain - and the findings have major consequences.
Those reactions quickly drown out the subtle signals from the stomach that
are saying, in effect, "Still digesting down here. Don't send more!"
Social cues add pressure and permission to indulge. Soon, everybody is
having a slice—or two.
Scholars have understood the different motives for eating as far back as
Socrates, who counseled, "Thou shouldst eat to live, not live to eat." But
nowadays, scientists are using sophisticated brain-imaging technology to
understand how the lure of delicious food can overwhelm the body's
built-in mechanism to regulate hunger and fullness, what's called
"hedonic" versus "homeostatic" eating.
in the brain - and the findings have major consequences.
Those reactions quickly drown out the subtle signals from the stomach that
are saying, in effect, "Still digesting down here. Don't send more!"
Social cues add pressure and permission to indulge. Soon, everybody is
having a slice—or two.
Scholars have understood the different motives for eating as far back as
Socrates, who counseled, "Thou shouldst eat to live, not live to eat." But
nowadays, scientists are using sophisticated brain-imaging technology to
understand how the lure of delicious food can overwhelm the body's
built-in mechanism to regulate hunger and fullness, what's called
"hedonic" versus "homeostatic" eating.
One thing is clear: Obese people or those with existing weight issues react much more hedonistically to sweet, fat-laden food in the pleasure and reward circuits of the brain than
healthy-weight people do. Simply seeing pictures of tempting food can
light up the pleasure-seeking areas of obese peoples' brains.
Two Reactions to Cake
Two conferences on obesity examined aspects of how
appetite works in the brain and why some people ignore their built-in
fullness signals. (How many times have you said "I'm SO full and then polished off dessert ?!) Scientists hope that breakthroughs will lead to ways to retrain people's thinking about food or weight-loss drugs that can target certain brain areas.
In a study presented this week at the International Conference on Obesity
in Stockholm, researchers from Columbia University in New York showed
pictures of cake, pies, french fries and other high-calorie foods to 10
obese women and 10 non-obese women and monitored their brain reactions on
fMRI scans. In the obese women, the images triggered a strong response in
the ventral tegmental area (VTA), a tiny spot in the midbrain where
dopamine, the "desire chemical," is released. The images also activated
the ventral pallidum, a part of the brain involved in planning to do
something rewarding.
"When obese people see high-calorie foods, a widespread network of brain
areas involved in reward, attention, emotion, memory and motor planning is
activated, and all the areas talk to each other, making it hard for them
to resist," says lead investigator Susan Carnell, a research psychiatrist
at the New York Obesity Research Center at Columbia University.
The Power of Cake? Take the Quiz !
This Power-of-Food Scale helps gauge how vulnerable you are to 'hedonic'
eating. Indicate from 1-5 which of the following best describes you:
1 Don't agree at all
2 Agree a little
3 Agree somewhat
4 Agree
5 Strongly agree
eating. Indicate from 1-5 which of the following best describes you:
1 Don't agree at all
2 Agree a little
3 Agree somewhat
4 Agree
5 Strongly agree
___ 1. I find myself thinking about food even when I'm not physically
hungry.
___ 2. I get more pleasure from eating than I do from almost anything else.
___ 3. If I see or smell a food I like, I get a powerful urge to have some.
___ 4. When I'm around a fattening food I love, it's hard to stop myself
from at least tasting it.
___ 5. It's scary to think of the power that food has over me.
___ 6. When I know a delicious food is available, I can't help myself from
thinking about having some.
___ 7. I love the taste of certain foods so much that I can't avoid eating
them even if they're bad for me.
___ 8. Just before I taste a favorite food, I feel intense anticipation.
___ 9. When I eat delicious food I focus a lot on how good it tastes.
___ 10. Sometimes, when I'm doing everyday activities, I get an urge to eat
"out of the blue" (for no apparent reason).
___ 11. I think I enjoy eating a lot more than most other people.
___ 12. Hearing someone describe a great meal makes me really want to have
something to eat.
___ 13. It seems like I have food on my mind a lot.
___ 14. It's very important to me that the foods I eat are as delicious as
possible.
___ 15. Before I eat a favorite food my mouth tends to flood with saliva.
Scoring: Add up your responses and divide the total by 15.
1.0 - 2.3: You're unlikely to be preoccupied with food or lose control over
eating.
2.4 - 3.6: You're somewhat preoccupied with food but are unlikely to have a
problem unless you're significantly overweight.
3.7 - 5.0: You're frequently preoccupied with food and at risk of losing
control over your eating. This is especially problematic if you are also
significantly overweight.
Similar brain reactions occurred in the obese subjects even when
researchers merely said the words "chocolate brownie"—but not when they
saw or heard about lower calorie foods such as cabbage and zucchini.
Reactions were far less pronounced in the non-obese subjects.
More such studies are being presented in Pittsburgh this week at the
annual meeting of the Society for the Study of Ingestive Behavior. In one,
neuroscientists from Yale University's John B. Pierce Laboratory had 13
overweight and 13 normal-weight subjects smell and taste chocolate or
strawberry milkshakes and observed their brains with fMRI scans.
The overweight subjects had strong reactions to the food in the
amygdala—the emotional center of the brain—whether they were hungry or
not. The healthy-weight subjects showed an amygdala response only when
they were hungry.
"If you are of normal weight, your homeostatic mechanisms are functioning
and controlling this region of the brain," says lead investigator Dana
Small. "But in the overweight group, there is some sort of dysfunction in
the homeostatic signal so that even though they weren't hungry, they were
vulnerable to these external eating cues."
Studies have found that a diet of sweet, high-fat foods can indeed blunt
the body's built-in fullness signals. Most of them emanate from the
digestive tract, which releases chemical messengers including
cholecystokinin, glucagon-like peptide and peptide YY when the stomach and
intestines are full. Those signals travel up to the brain stem and then
the hypothalamus, telling the body to stop eating.
Obesity also throws off the action of leptin, a hormone secreted by fat
tissue that tells the hypothalamus how much energy the body has stored.
Leptin should act as a brake against overeating, and it does in
normal-weight people. But most obese people have an overabundance of
leptin, and somehow their brains are ignoring the signal.
All these findings beg the question, which came first? Does obesity
disrupt the action of leptin, or does a malfunction in leptin signaling
make people obese?
Similarly, are some people obese because their brains overreact to
tempting food, or do their brains react that way because something else is
driving them to overeat? Researchers at Yale and elsewhere are turning to
such questions next. "It's possible that these changes reflect how the
brain has adapted to eating patterns in obese people, and that could
create a vicious circle, putting them at risk for even more disordered
eating," says Dr. Small.
There are plenty of other metabolic mysteries, too: Why are some "foodies"
who get intense pleasure from eating able to stop when they're full and
others aren't? Is the tendency to eat way past fullness genetic or learned
behavior, and how much can it be changed?
The answers are still elusive, but neuroscientists and behavioral experts
are finding some tantalizing clues.
Some fMRI studies have found that while tempting food stimulates the
release of dopamine in obese people, they actually have fewer dopamine
receptors than normal weight subjects do, so they may derive less pleasure
from actually eating, setting up a craving for more.
Curiously, several studies have shown that some forms of gastric bypass
surgery can actually create changes in the brains of formerly obese people
—and not just because their stomachs are smaller and fill up more quickly.
Levels of leptin and glucose tend to drop in bypass patients, ending
diabetes for many of them. PET scans also show that bypass patients have
more dopamine circulating in their brains, which may help control appetite
as well.
Some of the most intriguing imaging studies have peered into the brains of
people who have lost significant weight and kept it off through diet and
exercise alone—although researchers say they're hard to find.
"They are very controlled individuals, and they are very rare. We had to
fly some in from Alaska," says Angelo Del Parigi, a neuroimaging
scientists who finally located 11 "post-obese" subjects who had dieted
down to the lean range. In his studies for the National Institutes of
Health's diabetes research center in Phoenix, Dr. Del Parigi found that
food still elicited strong responses in the middle insula and the
hippocampus, brain areas involving addiction, reward, learning and memory,
just like the 23 obese subjects did.
"The only way they have to counteract these strong predispositions
is by having a very controlled lifestyle, with restrained food intake and
exercise." He and his colleagues at the NIH have observed that in PET scans, too. In
another study, 17 people who had successfully lost weight had more
activity in the dorsal lateral prefrontal cortex, a part of the brain
involved in impulse-control in response to food than people who were still
obese.
In short, successful weight losers seemed to have having second thoughts
about eating on impulse, says Dr. Del Parigi. "These people see a piece of
pie that is very inviting, but they think, 'No, I have to diet. Otherwise,
I will become obese again. I will suppress that pleasure,' " he says.